Compose file versioning in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the version field in a Docker Compose file?

The version field specifies the Compose file format version, which determines the syntax and features available in the file.

intermediate

Name two common Compose file versions and one key difference between them.

Version '2' supports networking and volumes with some limitations, while version '3' adds support for Docker Swarm and deploy configurations.

beginner

How do you specify the Compose file version in a docker-compose.yml file?

At the top of the file, add a line like: version: '3.8' to specify version 3.8 of the Compose file format.

intermediate

What happens if you omit the version field in a Docker Compose file?

Docker Compose assumes version 1 format, which has limited features and is deprecated in newer Docker Compose versions.

beginner

Why should you choose the latest stable Compose file version compatible with your Docker Engine?

Using the latest stable version ensures access to new features, better performance, and improved compatibility with Docker Engine capabilities.

What does the 'version' field in a Docker Compose file control?

A The container image version

B The Docker Engine version to use

C The Compose file format and features available

D The network driver version

The 'version' field defines the Compose file format version, which controls syntax and available features.

Which Compose file version introduced support for Docker Swarm deploy configurations?

A Version 2

B Version 3

C Version 1

D Version 4

Version 3 added support for Docker Swarm and deploy configurations.

If you omit the version field in a Compose file, what version is assumed?

A Version 1

B Version 3

C Version 2

D No version is assumed

Omitting the version field defaults to version 1 format.

How do you specify Compose file version 3.8 in your docker-compose.yml?

A version = 3.8

B compose_version: 3.8

C file_version: 3.8

D version: '3.8'

The correct syntax is: version: '3.8' at the top of the file.

Why is it important to use a Compose file version compatible with your Docker Engine?

A To avoid syntax errors and use supported features

B To speed up container startup time

C To reduce image size

D To enable GPU support

Compatibility ensures the Compose file syntax and features work correctly with your Docker Engine.

Explain the role of the version field in a Docker Compose file and how it affects your configuration.

Think about how different versions enable different features.

Describe the differences between Compose file versions 2 and 3 and when you might choose one over the other.

Consider the use of Docker Swarm and advanced deployment features.

Practice

1. What does the version field in a Docker Compose file specify?

easy

A. The version of the operating system hosting Docker

B. The version of the Docker Engine installed on the system

C. The version of the application running inside the container

D. The format and features Docker Compose should use to read the file

Solution

1. Step 1: Understand the role of the version field

   The version field tells Docker Compose which syntax and features to expect in the file.

2. Step 2: Differentiate from other version types

   This field is not about Docker Engine, app, or OS versions but about Compose file format.

3. Final Answer:

   The format and features Docker Compose should use to read the file -> Option D

4. Quick Check:

   Compose file version = file format and features [OK]

Hint: Version field controls Compose syntax, not Docker or app versions [OK]

Common Mistakes:

• Confusing Compose file version with Docker Engine version
• Thinking it sets app container version
• Assuming it relates to OS version

2. Which of the following is the correct way to specify the Compose file version in YAML?

easy

A. version: "3.9"

B. version : 3.9

C. version = "3.9"

D. "version" : 3.9

Solution

1. Step 1: Recall YAML syntax for key-value pairs

   YAML uses colon and space to assign values, and strings should be quoted if they contain dots.

2. Step 2: Identify correct quoting for version

   The version number with a decimal should be quoted as a string: "3.9".

3. Final Answer:

   version: "3.9" -> Option A

4. Quick Check:

   YAML version field quoted with colon = correct [OK]

Hint: Use colon and quotes for version like version: "3.9" [OK]

Common Mistakes:

• Using equals sign instead of colon
• Not quoting version with decimals
• Putting quotes around key instead of value

3. Given this Compose file snippet:

version: "2.4"
services:
  web:
    image: nginx
    ports:
      - "80:80"

What will happen if you run this with Docker Compose version 1.27.0?

medium

A. It will run successfully using version 2 features

B. It will fail because version 2.4 is not supported in 1.27.0

C. It will ignore the version and run with default settings

D. It will upgrade the Compose file version automatically

Solution

1. Step 1: Check Docker Compose 1.27.0 compatibility

   Docker Compose 1.27.0 supports Compose file version 2.4 syntax and features.

2. Step 2: Understand behavior on supported versions

   Since version 2.4 is supported, Compose will run the services as defined without errors.

3. Final Answer:

   It will run successfully using version 2 features -> Option A

4. Quick Check:

   Supported Compose version runs fine [OK]

Hint: Check Compose version supports file version before running [OK]

Common Mistakes:

• Assuming newer Compose versions are always required
• Thinking Compose auto-upgrades file versions
• Believing unsupported versions cause silent ignores

4. You have this Compose file:

version: "3.8
services:
  app:
    image: myapp
    ports:
      - "8080:80"

When running docker-compose up, you get an error. What is the likely cause?

medium

A. The ports mapping syntax is incorrect

B. The image name is invalid without a tag

C. The version number is not quoted, causing a YAML parsing error

D. Docker Compose does not support version 3.8

Solution

1. Step 1: Identify YAML syntax issue

   The version field with a decimal must be quoted as a string to avoid YAML parsing errors.

2. Step 2: Check other fields for errors

   Image name without tag is valid (defaults to latest), ports syntax is correct, and version 3.8 is supported.

3. Final Answer:

   The version number is not quoted, causing a YAML parsing error -> Option C

4. Quick Check:

   Unquoted decimal version causes YAML error [OK]

Hint: Always quote version numbers with decimals in Compose files [OK]

Common Mistakes:

• Not quoting version with decimals
• Assuming image needs explicit tag
• Misreading ports syntax as error

5. You want to use a new Compose feature available only in version 3.9, but your current Compose file uses version "3.7". What is the best way to update your Compose file to use the new feature safely?

hard

A. Add the new feature without changing the version field, it will work automatically

B. Change the version field to "3.9" and test the file with your Docker Compose version

C. Downgrade your Docker Compose to an older version that supports 3.7 features only

D. Remove the version field entirely to let Docker Compose decide

Solution

1. Step 1: Update the version field to match the new feature

   Set the version field to "3.9" to indicate use of new features introduced in that version.

2. Step 2: Test compatibility with your Docker Compose version

   Ensure your Docker Compose installation supports version 3.9 to avoid errors.

3. Final Answer:

   Change the version field to "3.9" and test the file with your Docker Compose version -> Option B

4. Quick Check:

   Update version field and verify Compose support [OK]

Hint: Match version field to features and test Compose version support [OK]

Common Mistakes:

• Using new features without updating version field
• Removing version field causing unpredictable behavior
• Ignoring Compose version compatibility